Process of coating metallic aluminum or aluminum alloys with aluminum oxide skin



Patented Nov. 12, 1929 TSUNETABO KUIIRAI, OI HAKA'IO-IAGBI, TOYOTAIA-GO PATENT OFFICE AHDSAIAIUmOI OJ'I-IAGHI, KITATOSHII A-GORL TOKYO rmrcruna, JAPAN, ASSIGITOBS TO ZAIDAH HOJIN I0, OI BONGO-1U, TOKYO, JAPAN IBOGFBS CI coa'rm G mam-1c AI-UIIHUI 03 um ALLOYS WITH ALUIIHUI OIEDB am He Drawing. Application fled December 2,

This invention relates to a sroeem of coat ing metallic aluminum or uminum alloy with aluminum oxide skin of desired thickness and consists in carrying out electrolysis b direct or alternating currentwith the meta; or its alloy as electrode or electrodes,'and using electrolyte of an aqueous solution of special organic acids such as oxalic and malonic acid or their salts, so that the skin produced on the surface of the metal or its alloy will not check the current flowing therethrough. The object of the invention is to cover the metal or its alloy with aluminum oxide skin which is of desired thickness and has an excellent property of electric insulation and anticorrosion, together with great mechanical strength and hardness, in an easy and effective manner. An aqueous SOllltlOll' of special organic oxidizing agents such as oxalic acid, malonic acid or their sodium, potassium or calcium salts may also be used for the electrolyte .of the invention.- Moreover, either direct or alternating-current may be used for electrolysis. When 'rect current is used, the positive electrode is made of metallic aluminum or its alloy, while the negative electrode may be any other conductor inactive to the electrolyte. When alternating current is used, both electrodes can be made of the metal or its alloy. Concentration of electrolytes, current voltage and density, and the time of electrolysis may vary accor g to kinds of electrolyte and thickness of the skin The following examples will fully explain the invention.

Example I .A.queous solution of 1.0 to 3.0 per cent of oxalic acid or its salts is used as the electrolyte. When alternating current is used for electrolysis both electrodes are made of metallic aluminum or aluminum alloys, such as duralumin. When direct current is used, the positive electrode is made of metallic aluminum or aluminum alloy, while the negative electrode may be metallic aluminum or any other conductor inactive to the electrolyte.

.In both cases the electrolysis should preferably be performed under the working voltage of to 100 volts and the current density 1984, Serial Io. 758,525, and in Japan December 19, 1988.

of 0.05 to 0.03 amperes per square centimeter. After electrolysis of about one hour, the surface of aluminum or its alloy is covered with the skin having yellow or gray enamel like luster, which has electrically insulating and antieorrosive pro 'es.

The thickness of this skin increases nearly proportional to the quantity of electricity use for electrolysis.

Example II.A ueous solution of 1.0 to 3.0 per cent of m onic acid or its salts is used as the electrolyte under the similar condition and process of electrolysis as explained in the Example I, the surface of aluminum or its alloy is covered with the skin having gray enamel like luster with insulatin and anti-corrosive properties.

all cases, the temperature of the electrolyte is preferably to remain under 30 C. by cooling and continuous agitation is recommended.

It is a well known fact that the metallic aluminum has a specialproperty of checking the flow of electric current when it is used'as the anode on electrolysis. For example, when it is used as the electrode in the so ution of silicates, boric acid, borates, phosphates, chromic acid, manganic acid or their salts, or of organic acids such as, succinic and citric acids; the flow of current is immediately checked by the formation of thin film or skin on the surface of the metallic aluminum.

Many attempts have been carried out to make electric insulating and anticorrosive skin on the surface of metallic aluminum by the electrol 'c process in the solution above mentioned, ut these have failed to get a sufiiciently thick film to withstand several hundred volts, owing to the properties of checking the current by'the thin film thus produced.

Accordin to the present invention, however, either y addin a small quantity of inorganic acids or their salts, in the solution of inorganic oxidizinfialgents or by choosing solutions of special ds of or anic acids, the current flows'for suflicient uration, so that a film of an desired thickness is produced on the sur ace of the aluminum or its alloy.

2 v v mamas In the claims the expressionfiluminum material is intended to cover either sub stantially pure aluminum or an aluminum alloy.

Claims:

1. In the process of electrolytically forming an insu ating and anti-corrosive oxide coating on aluminum material, acting as an electrode, the ste which comprises electrolyzing a solution 0 a dibasic organic acid compound in contact with said material.

2. In the process of electrolytically forming an insu ating and anti-corrosive oxide coating on aluminum material, acting as an electrode the step which comprises electrolyzing a solution of an oxalic acid compound in contact with said material.

3. The process of'electrolytically forming an insulating and anti-corrosive oxide coating on aluminum acting as an electrode which comprises electrolyzing a solution of an oxalic acid compound in contact with said aluminum with a voltage of to volts and a current density of .05 to .03 amperes per square centimete'r.

4. The process as in claim 3 wherein the solution contains from 1 to 3 per cent of the oxalic acid compound.

In testimony whereof they aflix their signatures.

TSUNETARO KUJIRAI. SAKAE UEKI. 

